Reversal type electroscopic developer powder



United States Patent REVERSAL TYPE ELECTROSCOPIC DEVELOPER POWDER HenryWielicki, Philadelphia, Pa., assignor to Radio Corpol-anon of America, acorporation of Delaware No Drawing. Filed Mar. 28,1958, Ser. No. 724,522

11 Claims. (Cl. 252-62.1)

This invention relates to electrostatic printing and particularly, butnot exclusively, to improved reversal type electroscopic developerpowders for electrostatic printing and to improved methods ofelectrostatic printing employing such developer powders. 7

An electrostatic printing process is that type of process for producinga visible record, reproduction or copy which includes as an intermediatestep, converting a light image or electrical signal into anelectrostatic charge pattern on an electrically-insulating layer. Theprocess usually includes the conversion of the charge pattern into avisible image which may be a substantially faithful reproduction of anoriginal, except that it may be a different size.

A typical electrostatic printing process may include producing anover-all, negative, electrostatic charge on the surface of aphotoconductive material such as selenium, anthracene or zinc oxidedispersed in an insulating binder. A light image is focused on thecharged surface, discharging the portions irradiated by the light rays,while leaving the remainder of the surface in a charged condition, tothus form an electrostatic image. The electrostatic image is renderedvisible by applying a developer power which is held electrostatically tothe charged areas of the surface. The powder image thus formed may befixed directly to the photoconductive material or it may be transferredto another surface upon which the reproduced image may be desired andthen fixed thereon. The fixing step commonly comprises fusing thedeveloper powder to the photoconductive material by the applicationthereto of heat. For a more detailed description of electrostaticprinting reference is made to Electrofax- Direct ElectrophotographicPrinting on Paper, by C. J. Young and H. G. Greig, RCA Review, volume15, No. 4.

It is often desirable in any type of printing to produce a reverse copyof an original. By this is meant to prodduce a negative copy from apositive original or, on the other hand, a positive copy from a negativeoriginal. In electrostatic printing, image reversal can be accomplishedby applying to the image a developer powder which is repelled by thecharged areas of the image and adheres to the discharged areas.Developer powders of this type have heretofore possessed manyundesirable characteristics. When an electrostatic image is developedwith many of these developer powders spurious deposits in charged areasof the image result. Also, when such powders are sufficiently fine forproper image development, they generally tend to form into balls orclots which result in smears or streaks in the developed image. Whenfairly large discharged areas exist in an electrostatic image, most suchpowders tend to adhere more strongly at the edges of the dischargedareas than near the center thereof resulting in an undesirable lineeffect in the developed image.

Accordingly, it is a general object of this invention to provideimproved reversal type developer powders for electrostatic printing.

Another object is to provide improved reversal type developer powderswhich will not deposit in unwanted areas of an electrostatic image.

Yet another object of this invention is to provide improved reversaltype developer powders which are sufficiently fine for proper imagedevelopment but which 7 will not tend to form into balls or clots.

' improved methods of electrostatic printing for producing a reversevisible image from an electrostatic charge image.

The foregoing objects and other advantages are accomplished inaccordance with this invention which provides improved reversal typedeveloper powders comprising particles of electroscopic material coatedwith finelydivided colloidal silica. For proper development ofelectrostatic images on negatively charged surfaces, it is importantthat the electroscopic material have a positive triboelectricrelationship with respect to the colloidal silica. It is also importantthat the developer powder be made up of electroscopic material andcolloidal silica in proper proportions. Generally, a ratio of about 1.5to 9 parts by weight of colloidal silica and 100 parts by weight ofelectroscopic material is satisfactory. The exact proportions willdepend to a large extent on the particle size of both the electroscopicmaterial and the colloidal silica. When desired a minor proportion of asuitable coloring agent may be incorporated in the developer powder toimpart thereto a desired color.

The improved methods of this invention relate to electrostatic printingprocesses wherein an electrostatic image is produced consisting ofnegative electrostatic charge areas on an insulating surface. Theimproved developer powder of this invention is applied to the insulatingsurface to produce a visible image. The powder is repelled from thenegatively charged areas and deposited in the other areas on theinsulating surface to produce a visible reverse image of theelectrostatic image. Application of the developer powder may beaccomplished in many ways such as, for example, cascade development ormagnetic brush development both of which are described in the Young andGreig publication, op. cit.

In a preferred embodiment of the invention the particles ofelectroscopic material are comprised of certain natural or syntheticresins, waxes or other low melting materials or mixtures thereof. Amaterial is normally selected which has a melting point less than thetemperature at which paper will char. A preferred temperature range isbetween 90 C. and 250 C. Such a material can be readily fused to aphotoconductive surface by the application thereto of heat. Whenparticles of such a material are coated with finely divided colloidalsilica, a developer powder is provided which adheres to the dischargedareas of a negatively charged electrostatic image and does not adhere tothe charged areas thereof.

Another preferred embodiment, particularly suited to color electrostaticprinting, contemplates a developer powder including a particulate zincoxide the particles of which have a first coating thereon of athermoplastic, film-forming material and a second coating over the firstof colloidal silica. In this embodiment the film-forming material isselected to have a positive triboelectric relationship with respect tothe colloidal silica. The filmforming material has a melting pointsubstantially within a range of from 90 C. to 250 C. and a viscositywithin a range of from 45 to 10,000 centipoises. The zinc oxide and thefilm-forming coating thereon are combined in a ratio of about 50 toparts by weight of zinc oixde to 15 to 50 parts by weight of thefilm-forming, coating material. The zinc oxide and the thermoplasticcoating together comprise the electroscopic material in this embodiment.

When particles of zinc oxide are included in a reversal type developerpowder they are characterized by one of two properties. These particlesmay be capable of holding an electrostatic charge in darkness or theymay be relatively conductive. The first property is found in a class ofzinc oxides known as French process zinc oxides. The second property isfound in the class known as American process zinc oxides. When Frenchprocess zinc oxides are employed, a developed powder image is obtainedwhich in subsequent printing procedures may be overprinted with anotherpowder image. When American process zinc oxides are employed, a powderimage is obtained which is incapable of being overprinted in subsequentprinting procedures. When French process zinc oxides are used, dyesensitizing agents may be added to the developer powder to change thespectral response of the zinc oxide.

Specific examples and additional advantages of the reversal typedeveloper powders and of the improved methods of electrostatic printingof this invention are included in the detailed description whichfollows.

ELECTROSCOPIC MATERIALS Many natural or synthetic resins and waxes canbe provided in particulate form and coated with colloidal silica toprovide reversal type developer powders in accordance with thisinvention. Examples of suitable materials which are triboelectricallypositive with respect to colloidal silica include the following:

(1) Piccolastic 4358A (a thermoplastic resin composed of polymers ofstyrene, substituted styrene and its homologs, made by the PennsylvaniaIndustrial Chemical Corporation, Clairton, Pennsylvania).

(2) Carnauba wax.

(3) Polymekon Wax (a chemically modified microcrystalline wax of theWarwick Wax Co., New York, N.Y.).

(4) Ultracera Amber Wax (a microcrystalline petroleum wax of the BarecoOil Co., Barnsdall, Oklahoma).

(5) BE Square Wax White (a microcrystalline petroleum wax of the BarecoOil Co.).

(6) Petronauba D Wax (a. microcrystalline petroleum wax of the BarecoOil Co.).

(7) Piccolyte S-135 (a thermoplastic hydrocarbon terpene resin of thePennsylvania Industrial Chemical Corp, Clairton, Pennsylvania).

(8) A mixture of Polymekon Wax and Piccolyte 8-1 15.

(9) A mixture of Acrawax C (a synthetic wax-octadecenamide, of the GlycoProducts Co., Brooklyn, New York) and calcium stearate.

(10) A mixture of Acrawax C and a solid silicone resin (such as DowCorning Silicone No. R-5071).

(11) A mixture of Piccolastic D-100 and Piccolastic 6-125.

COLLOIDAL SILICA Colloidal silicas which satisfy the requirements ofthis invention consists of at least 90% pure S10 with the remaining 10%made up of such impurities as Na SO A1 and Fe O These colloidal silicasgenerally have a particle size ranging from .015 to microns meandiameter. Two specific examples of colloidal silicas include Santoceland Cab-O-Sil. Santocel is a colloidal silica manufactured by theMonsanto Chemical Co., Inorganic Division, St. Louis, Missouri, andcomprises from 90 to 93% SiO 1 to 5% volatile substances, 2 to 3% Na SOand about 1% A1 0 plus Fe O It has an average particle size Within arange of from 0.5 to 5 microns in diameter. Cab-O-Sil is a. colloidalsilica manufactured by the Godfrey L. Cabot Co., Cambridge,Massachusetts, comprising between 99.0 and 99.7% SiO 0.2 to 2.0% freemoisture and about .004% Fe O It 4 has a particle size range of from.015 to .020 micron mean diameter.

In general, the reversal type toner powder is prepared by thoroughlymixing 1.5 to 9 parts by weight of colloidal silica with 100 parts byweight of electroscopic particles. The optimum ratio of colloidal silicato electroscopic particles appears to be about 5 parts by weight ofcolloidal silica to 100 parts by weight of particles. It is preferred toadd the colloidal silica in small quantities and to follow each additionby ball milling the mixture for approximately 30 minutes. The optimumball milling time is from 3 to 5 hours.

The incorporation of proper amounts of colloidal silica in the developerpowders of this invention imparts additional advantageous propertiesthereto. Taekiness is substantially eliminated and the free flowingproperty of the developer powders substantially improved. Alsosubstantially eliminated is any tendency for the powders to form intoballs or clots.

COLORING AGENTS Various coloring agents may be added singly or incombination to the foregoing toner powders to provide any desired color.Color developer powders will generally include from .2 to 12 parts byweight of a coloring agent for each 100 parts by weight of electroscopicparticles. Suitable coloring agents include the following:

(1) Carbon black (2) Fluorescein Sodium (Color Index No. 766) (3) EosinY (Color Index No. 768) (4) Rose Bengal (Color Index No. 779) (5)Brilliant Green (Color Index No. 662) (6) Patent Blue (Color Index No.672) (7) Thioflavin TG (Color Index No. 49005 A specific example of adeveloper powder in accordance with this invention includes thefollowing.

This developer powder is prepared in the manner described above.

ZINC OXIDES In accordance with the second embodiment of this inventionreversal type developer powders are provides with comprise electroscopicparticles consisting of particulate zinc oxide coated with low meltingthermoplastic electroscopic material and overcoated with colloidalsilica. The zinc oxides contemplated herein fall into two classes, theFrench process zinc oxides which permit overpn'nting in subsequentelectrostatic printing procedures and the American process zinc oxideswhich are incapable of such overprinting. The determination as to whichspecific class a zinc oxide belongs is important. Various methods havebeen devised to make this determination.

Method 1.A mixture is prepared comprising about 10 milligrams of dryzinc oxide powder and a few drops of an solution of silicone resin inxylene (G.E. SR82, marketed by the General Electric Co., SiliconeProducts Division, Waterford, N.Y) diluted with tolacne in the ratio 60grams solution to 105 grams toluene. The mixture is coated on filterpaper and dried to produce a dry coating over an area about 0.25 inch indiameter. The dry coating is cooled to about -l C. and examined in lightfrom a mercury vapor lamp having a maximum output at about 3650 A. Thezinc oxides (French process) which produce printable coatings vproduce alavender or orange luminescence. Other zinc oxides (American process)exhibit a green or yellow luminescence.

Method 2.--About 0.25 gram of dry zinc oxide powder is placed in asilica boat. The boat is inserted into a silica tube and the systemflushed with hydrogen gas. The tube and boat are fired for about 5minutes at about 1000 C. in a stagnant hydrogen atmosphefe. The boat iscooled in hydrogen to room temperature. The fired zinc oxide is examinedin light from a mercury vapor lamp having a maximum output at about 3650A. The zinc oxides (French process) which produce printable coatingsluminesce brightly. Other zinc oxides (American process) luminesceweakly or not at all.

A preferred overprinting zinc oxide is one which pro: duces a lavendercolor in Method 1 and luminesces brightly in Method 2. A preferrednon-overprinting zinc oxide is one which exhibits a green luminescencein Method 1 and which does not luminesce at all in Method 2.

-It has been found that the useful overprinting zinc oxides selected inaccordance with the above procedures have a surface photoconductivity ofat least about ohm- /square/watt/cm. when subjected to a light of awavelength of about 3900 A. The surface photoconductivity of a zincoxide can be determined by applying thereto a third method as follows:

Method 3.--A small quantity of zinc oxide is reduced to a powder andcompressed under high pressure (about 15,000 lbs. per square inch) toform a pellet. Electrodes, as of silver paste, are applied on thesurface of the pellet leaving a square area of surface uncoated. Thepellet is then placed in a monochromator with the aforementioneduncoated surface area facing the light source and successive wavelengthsof light throughout the spectrum are projected on this surface. Thelight beam projected onto the surface is chopped at about 23.5 c.p.s. bya constant speed rotating disc, pierced to produce equal intervals oflight and darkness. A DC. potential is placed across the electrodes andthe current flowing between the electrodes is measured as a function ofwavelength with the intensity of radiation being held constant.

The zinc oxides which are suitable are those which are substantiallyelectrically non-conductive in the dark. When exposed to light, theyshould exhibit a surface photoconductivity of a certain level in orderto be of practical use for the purposes of this invention. In testingzinc oxides to determine their suitability and utilizing a pellet form,it is convenient to express the results of the measurements of the testas surface photoconductivity because substantially all of the light isabsorbed in a thin layer at the surface of the pellet. It has been foundthat, to be useful for overprinting in this invention, the zinc oxideselected shrould have a surface photoconductivity of at least 10-ohmsquare/ watt/cm. when exposed to a wavelength of about 3900 A. Inaddition to the foregoing it has also been found that those zinc oxideswhich are suitable for use in non-overprinting developer powder possessanother determinable property. Such zinc oxides must have a volumeresistivity such that when combined with a suitable film-forming coatingand fused, the fused mixture of zinc oxide and coating material willhave a volume resistivity of 10 ohm-cm. or less.

FILM FORMING MATERIALS as a binder holding the zinc oxide on theinsulating sur-' face. The viscosity of the coating material comprisesanother important criterion. The viscosity must be low enough so that,when melted, the coating will flow off the zinc oxide particles leavingthem partially exposed with only a thin film remaining on the exposed orprotruding portions of the zinc oxide particles. After the film-formingmaterial has been melted and fused to an insulating surface, it isimportant that the protruding particles of zinc oxide shall present amatte surface, i.e., the film of the coating material remaining on theprotruding portions of zinc oxide must not be thick enough to provide agloss finish. Were the film of coating material to have such athickness, the particles of zinc oxide might be insulated to such adegree as to cause the coating material to retain an electrostaticcharge and thereby impair subsequent printing operations. It isextremely difiicult to measure the thickness of such a film, however, ifa physical appearance substantially like that described is achieved, thecoated electroscopic particles will have the characteristics andproperties contemplated in this invention. It is also preferred that thefilm-forming material should not be so free flowing as to allow it tomigrate into unwanted areas of the surface when melted. A preferredviscosity range is from 45 to 10,000 centipoises as measured with adirect reading Brookfield viscosimeter with a spindle speed of 60 rpm.at a temperature just slightly above the melting point of the material.

Film-forming materials having the foregoing properties may comprisecertain natural or synthetic resins, waxes, or other low meltingmaterials or mixtures thereof. For example any of the followingmaterials or combinations of materials may be used:

Film-forming materials such as those specified may also includemodifying agents such as plasticizers, toughening agents, hardeningagents, or dispersing agents, which are added to obtain desired physicaland electrical properties.

A developer powder, in accordance with this embodiment of the invention,includes a ratio of zinc oxide to film-forming material Within a rangeof from 1 to 7 parts by weight of zinc oxide to one part by weight offilm-forming material. Generally, the film-forming material is meltedand finely-divided zinc oxide dispersed into the melt. The melt is thenallowed to cool and harden, after which it is broken up and reduced to adesired powder form. Finally, the colloidal silica is mixed into theabove powder in the manner described heretofore. The ratio of zinc oxideto film-forming material specified above is important in a givendeveloper powder formula. The exact ratio depends to a large extent onthe particle size and the dispersion of the zinc oxide chosen.

Coloring agents such as dyes, stains or pigments can be added to theforegoing powders to produce a desired color. The coloring agentsspecified heretofore may be employed singly or in any combination and inthe specified proportions.

In addition to the foregoing, various sensitizing agents may be employedto vary the spectral response of the photoconductive French process zincoxides. For example, photoconductive white zinc oxide has a spectralsensitivity having a peak in the near ultraviolet range. By addingvarious sensitizing agents a white photocon- Carnauba wax Polymekon WaxUltracera Amber Wax BE Square Wax White Petronauba D Wax Piccolyte 8-135A mixture of Polymekon Wax and Piccolyte S1l5 A mixture of Acrawax C andcalcium stearate A mixture of Acrawax C and a solid silicone resinductive zinc oxide can be provided having an additional sensitivity peakin other portions of the spectrum. Satisfactory sensitizing agentsinclude the following. 7

(l) Fluorescein Sodium (Color Index No. 766) (2) Eosin Y (Color IndexNo. 768) (3) Rose Bengal (Color Index No. 779) (4) Brilliant Green(Color Index No. 662) (5) Patent Blue (Color Index No. 672) (6)Thioflavin TG (Color Index No. 49005) 'OVERPRINTING DEVELOPER POWDERSThe following group of examples provide developer powders suitable foroverprinting procedures. These powders when fused to an insulatingsurface present a matte finish which is easily overprinted by otherdeveloper powders in subsequent electrostatic printing steps.

Example 11 WHITE DEVELOPER POWDER Parts by weight Carnauba wax 1 Frenchprocess zinc oxide 2 Colloidal silica 0.15

This is the simplest type of overprinting developer powder. The wax ismelted and particles of the zinc oxide having a particle size of from.025 to .5 micron mean diameter are added to the melt. Particle size andshape of the zinc oxide determine to some extent the ratio of zinc oxideto coating material. Due to the bulking characteristic of zinc oxidefiner particles usually require more coating material since there ismore total surface to be covered. Continuous stirring from 15 to 30minutes is sufiicient to thoroughly disperse the zinc oxide in the waxwhen the batch weighs about 100 grams. The mixture is then allowed tocool and harden after which it is reduced to a fine powder. This isaccomplished by ball milling the mixture for about 3 hours and thenclassifying it as to particle size. For most purposes, the fract-ionbelow 200 mesh (74 microns) is suitable for use as an electroscopicdeveloper powder. The classified particles are then intimately mixedwith the colloidal silica.

Example III BLUE DEVELOPER POWDER Preparation the same as in Example IIexcept that the calcium stearate is added to the melt before the zincoxide, and the coloring agent after the zinc oxide.

Example IV RED DEVELOPER POWDER Parts by weight Acrawax C 36 Siliconeresin (solid) 5 French process zinc oxide 80 Sudan III Red 4 Oil Yellow26 2 Colloidal silica 5 Preparation the same as in Example II except theAcrawax C and silicone resin are melted together before adding the zincoxide and coloring agent.

Example V GREEN DEVELOPER POWDER Parts by weight Piccolyte S-l35 Frenchprocess zinc oxide 30 Benzidine Yellow 1 Brilliant Oil Blue B.M.A. 0.23Colloidal silica 2.5

Preparation as in Example II except add the coloring agents after addingthe zinc oxide to the molten Piccolyte.

SENSITIZING AGENTS The following sensitizing agents may be employed, inthe proportions indicated, with any of Examples II to V. Thesesensitizing agents are added to the melt after the zinc oxide andcoloring agents have been added thereto.

Example VI Add Brilliant Green to any of Examples II to V in the ratioof about 0.1 part Brilliant Green to 70 parts by weight of zinc oxide.This provides a photoconductive developer powder having a secondsensitivity peak of approximately 6230 A. in the orange-red portion ofthe spectrum.

Example VII Add Patent Blue to Examples 11 to V in a ratio of about 0.05part Patent Blue to 70 parts by weight of zinc oxide. This provides aphotocond-uctive developer powder having a second sensitivity peak ofabout 6380 A. in the orange portion of the spectrum.

Example VIII Add Thioflavin T6 to Examples II to V in a ratio of about0.1 part of Thioflavin to about 70 parts by weight of zinc oxide. Thisprovides a photoconductive developer powder having a second sensitivitypeak at about 4080 A. in the blue portion of the spectrum.

Example IX Add Rose Bengal to Examples II to V in a ratio of about 0.05part of Rose Bengal to about 70 parts by weight of zinc oxide. Thisprovides a photoconductive developer powder having a second sensitivitypeak at about 5500 A. in the green portion of the spectrum.

Example X Add Fluorescein Sodium to Examples II to V in a ratio of about0.05 part of Fluorescein to about 70 parts by weight of zinc oxide. Thisprovides a photoconductive developer powder having a second sensitivitypeak at about 4770 A. in the blue-green portion of the spectrum.

Example XI added thereto the powder has a single sensitivity peak atabout 37-50 A. in the ultraviolet portion of the spectrum. When one ofthe dyes of Examples VI to XI is added, the developer powder stillexhibits a peak at about 3750 A. as well as an additional peak at thewavelength cited for each example. Thus, there is provided means forobtaining a spectral response at almost any portion of the spectrum fromultraviolet to red.

9 NON-OVERPRINTING DEVELOPER POWDERS Example XII WHITE DEVELOPER POWDERParts by weight Ultracera Wax 20 American process zinc oxide 25Colloidal silica 2.5

Prepared as in Example II.

Example XIII YELLOW DEVELOPER POWDER Parts by weight Petronauba D Wax 20American process zinc oxide 50 Benzidine Yellow 2.5 Colloidal silica 3.5

Prepared as in Example V.

Example XIV RED DEVELOPER POWDER Parts by weight Piccolyte S-135 20American process zinc oxide 30 Oil Red N-1700 3 Oil Yellow 2G 1.2Colloidal silica 2.5

Prepared as in Example V.

Example XV BLUE DEVELOPER POWDER Parts by weight Polymekon Wax 15Piccolyte 8-115 American process zinc oxide 50 Condensation Blue 1Colloidal silica 3.5

Prepared as in Example IV.

COLOR PRINTING The reversal type developer powders described in ExamplesII through XV find particular utility in the electrostatic reproductionof color images. The overprinting powders of Examples II through XI areadapted for use in a printing method wherein an image of one color issuperimposed upon an image of another color. One such method comprisesthe steps of: (1) uniformly charging a photoconductive surface in anegative polarity; (2) exposing the photoconductive surface to a lightimage to produce thereon an electrostatic image; (3) developing thelatent electrostatic image by applying thereto a developer powder suchas those of Examples II through XI to produce a visible powder image inthe discharged areas on the photoconductive surface; (3) applying heatto the developer powder to cause the film-forming coating thereon toflow toward the photoconductive surface thereby leaving particles ofzinc oxide protruding above the film-forming material and leaving only avery thin film of that material on the protruding portions of the zincoxide particles; (3) repeating steps (1) and (2) to produce a secondelectrostatic image on the photoconductive surface having thereon thefirst developed image; (4) applying a different colored developer powderto the discharged areas of the second electrostatic image to produce asecond visible image. When a French process zinc oxide is employed toproduce the first visible image, the second visible image will overlapthe first in those areas upon which light impinged during the secondexposure. The steps of this procedure may be carried out as many timesas desired to produce a composite image in substantially natural colors.When an American process zinc oxide, such as those included in ExamplesXII to XV, is employed, deposition of the second developer powder willonly occur in areas not covered by the first powder image. Again thesteps of the procedure may be carried out as many times as desired toproduce a composite color image in which the separate colors aredeposited in discrete contiguous areas on the photoconductive surface.

There have been described new and improved reversal type elecroscopicdeveloper powders which make possible direct printing of reverse imagesin black and White or in any desired colors.

What is claimed is:

1. A reversal type developer powder for electrostatic printingcomprising particles of electroscopic material coated with afinely-divided colloidal silica, said electroscopic material comprisinga low melting point organic solid having a positive triboelectricrelationship with respect to said colloidal silica, said developerpowder comprising about 100 parts by weight of said electroscopicmaterial and about 1 /2 parts to 9 parts by weight of said colloidalsilica.

2. The developer powder of claim 1 wherein said electroscopic materialcomprises a thermoplastic, film-forming resin having a melting pointsubstantially within a range of from 90 C. to 250 C.

3. The developer powder of claim 2 including therein a minor proportionof a coloring agent.

4. A reversal type developer powder for electrostatic printingcomprising particles of zinc oxide having an electroscopic coatingthereon of a thermoplastic material having a melting point substantiallywithin a range of from 90 C. to 250 C. and a viscosity substantiallywithin a range of from 45 to 10,000 centipoises, and an additionalcoating on said particles consisting essentially of finely-dividedcolloidal silica, said developer powder comprising 50 to parts by weightof said zinc oxide, 15 to 50 parts by weight of said thermoplasticmaterial, and 1% to 9 parts by weight of said colloidal silica.

5. The developer powder of claim 4 including a minor proportion thereinof a coloring agent.

6. A reversal type developer powder for electrostatic printingcomprising 50 to 85 parts by Weight of a particulate French process zinco'xide having a surface photoconductivity of about 10' ohm-/square/watt/cm. at a wavelength of about 3900 A., 15 to 50 part byweight of an electroscopic coating on the particles of said zinc oxideconsisting of a thermoplastic insulating material having a melting pointsubstantially within a range of from C. to 250 C. and a viscositysubstantially Within a range of from 45 to 10,000 centipoises at atemperature slightly above said melting point, and an additional coatingon said particles consisting essentially of 1 /2 to 9 parts by weight ofa finely-divided colloidal silica, said thermoplastic, insulatingmaterial having a positive triboelectric relationship with respect tosaid colloidal silica.

7. The developer powder of claim 6 including 0.2 to 12 parts by weightof a coloring agent.

8. The developer powder of claim 6 including 002 to 1.4 parts by weightof a dye sensitizing agent for said zinc oxide.

9. The developer powder of claim 8 including 0.2 to 12 parts by Weightof a coloring agent.

10. A reversal type developer powder for electrostatic printingcomprising: 50 to 85 parts by weight of a particulate American processzinc oxide; 15 to 50 parts by weight of a fusible coating on theparticles of said zinc oxide consisting of a thermoplastic,electroscopic material having a melting point substantially Within arange of from 90 C. to 250 C. and a viscosity substantially within arange of from 45 to 10,000 centipoises at a temperature slightly abovesaid melting point; said coating and said particles of zinc oxide, whenfused, having a volume resistivity not in excess of 10 ohm-cm; and anadditional coating on said particles consisting essentially of 1 /2 to 9parts by weight of a finely-divided colloidal silica; said thermoplasticmaterial having a positive triboelectric relationship with respect tosaid colloidal silica.

1 1. The developer powder of claim 10 including 0.2 to 12 parts byweight of a coloring agent.

References Cited in the file of this patent 12 Walkup et a1. May 12,1953 Grieg et a1. Feb. 21, 1956 Landrigan July 3, 1956 Sugarman Aug. 14,1956 Coulter Mar. 19, 1957 Moncriefi-Yeates June 17, 1958 Mayer Mar. 10,1959 Insalaco June 30, 1959 Steppe Aug. 25, 1959 OTHER REFERENCES Younget al.: Electrofax Direct Electrophotographic Printing on Paper, RCAReview, December 1954, vol.

XV, N0. 4.

6. A REVERSAL TYPE DEVELOPER POWDER FOR ELECTROSTATIC PRINTINGCOMPRISING 50 TO 85 PARTS BY WEIGHT OF A PARTICULATE FRENCH PROCESS ZINCOXIDE HAVING A SURFACE PHOTOCONDUCTIVITY OF ABOUT 10-9OHM-1/SQUARE/WATT/CM.2 AT A WAVELENGTH OF ABOUT 3900 A., 15 TO 50 PARTSBY WEIGHT OF AN ELECTROSCOPIC COATING ON THE PARTICLES OF SAID ZINCOXIDE CONSISTING OF A THERMOPLASTIC INSULATING MATERIAL HAVING A MELTINGPOINT SUBSTANTIALLY WITHIN A RANGE OF FROM 90*C. TO 250*C. AND AVISCOSITY SUBSTANTIALLY WITHIN A RANGE OF FROM 45 TO 10,000 CENTIPOISESAT A TEMPERATURE SLIGHTLY ABOVE SAID MELTING POINT, AND AN ADDITIONALCOATING ON SAID PARTICLES CONSISTING ESSENTIALLY OF 11/2 TO 9 PARTS BYWEIGHT OF A FINELY-DIVIDED COLLOIDAL SILICA, SAID THERMOPLASTIC,INSULATING MATERIAL HAVING A POSITIVE TRIBOELECTRIC RELATIONSHIP WITHRESPECT TO SAID COLLOIDAL SILICA.